V8 testrunner is loading every test it has to run into memory greedily in order
to sort by slowness of the test case. The memory and CPU overhead for loading
the test-suites are non-trivial.

This CL restructures it by changing the sorting method.

R=machenbach@chromium.org
CC=​​sergiyb@chromium.org,yangguo@chromium.org

Bug: v8:8174
Change-Id: I08331182147b92cf4ac54823eea0e2b472f51e84
Reviewed-on: https://chromium-review.googlesource.com/c/1406684
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58821}